Automatic bleeder vent for covered floating roof tanks

ABSTRACT

Storage tank for a volatile liquid having an outer roof and an internal floating roof, a vent pipe having one end in liquid communication with the bottom space of the tank and the other end extending in vapor communication to outside the tank, means to support the floating roof above the tank bottom and above the liquid when the liquid contents drops below a predetermined level and port means communicating with the vent pipe in close proximity to the bottom of the floating roof for venting vapors from the space below the floating roof to the vent pipe and allowing air or another gas to enter this space from the outside of the tank when the floating roof is supported in nonfloating position above the liquid level. A float can be located in the vent pipe to float on the liquid. The float will be below the port means when the floating roof is not supported by liquid. When the floating roof is supported by the liquid, the float provides a barrier between liquid and vapor in the vent.

United States Patent 1 Strunc et a1.

[ 1 Jan. 28, 1975 6/1952 Moyer 220/44 R 9/1971 Wagoner et a1. 220/26 D [57] ABSTRACT Storage tank for a volatile liquid having an outer roof and an internal floating roof, a vent pipe having one end in liquid communication with the bottom space of the tank and the other end extending in vapor communication to outside the tank, means to support the floating roof above the tank bottom and above the liquid when the liquid contents drops below a predetermined level and port means communicating with the vent pipe in close proximity to the bottom of the floating roof for venting vapors from the space below the floating roof to the vent pipe and allowing air or another gas to enter this space from the outside of the tank when the floating roof is supported in nonfloating position above the liquid level. A float can be located in the vent pipe to float on the liquid. The float will be below the port means when the floating roof is not supported by liquid. When the floating roof is supported by the liquid, the float provides a barrier between liquid and vapor in the .vent.

Claims, 4 Drawing Figures 1 1 AUTOMATIC BLEEDER VENT FOR COVERED FLOATING ROOF TANKS [75] Inventors: Robert Winfield Strunc, Clarendon Hills; Robert Kenneth Doty, Western Springs, both of 111.

[73] Assignee: Chicago Bridge & Iron Company, Oak Brook, 111.

[22] Filed: Jan. 31, 1972 121] App]. No.: 222,158

[521 US. Cl ..?....2 /227, 137/587,;20/567 [51] Int. Cl Bd 87/18 [58] Field of Search 220/26 R, 26 S, 26 SA,

220/26 D, 26 E, B, 85 VR, 85 VS, 44 R; 137/202, 587, 429-433; 55/164, 165, 166, 169,

[56] References Cited UNITED STATES PATENTS .12 l l l PATENIEB JAN 2 81975 SHEET 10F 2 PATENTEB JAN 2 8 I975 SHEET 2 OF 2 AUTOMATIC BLEEDER VENT FOR COVERED FLOATING ROOF TANKS This invention relates to storage tanks for liquid products. More particularly, this invention is concerned with a vent for a storage tank for volatile liquid products which has an outer roof and an internal floating roof.

Storage tanks for volatile materials, such as liquid petroleum products, are in wide use. Such storage tanks generally are made of metal plate and comprise a metal bottom, a circular vertical metal wall and a roof which is generally either conical or domed, also made of metal. For the storage of volatile liquid petroleum products, and others, it is also conventional to utilize a floating roof inside of the tank to retard loss of product vapors. The floating roof rises and falls with the liquid level in the tank.

An enclosed storage tank of the described type having an internal floating roof generally is provided with means to support the floating roof above the tank bottom when the liquid contents in the tank drops below a predetermined level. The floating roof may be supported by auxiliary support means about two or three feet above the tank bottom when the liquid goes below such level in order that liquid inlets and outlets below the floating roof remain unobstructed. Legs can be placed beneath the floating roof to support the floating roof above the tank bottom or supporting flanges can be placed on the inside wall of the tank to hold it above the tank bottom a predetermined distance.

When the floating roof becomes supported by the auxiliary support means, it no longer floats on the liquid contents in the tank. This occurs when the liquid level in the tank drops below the position at which the floating roof becomes so supported and is not floating. When this happens, vapor from the liquid accumulates beneath the floating roof. The accumulation of vapor admixed with air beneath the so-supported floating roof is unavoidable. Subsequent filling of the tank causes the vapor and iar admixture to be expelled into the space above the floating roof and under the outer or external roof of the tank. The resulting vapor-air admixture can constitute an explosive composition which can be ignited through an electrical discharge into this space from a friction spark or some source of static electricity. If the admixture is ignited, severe damage could occur to the tank. There is accordingly a need for a means to remove the vapor from beneath the floating roof when it is supported by auxiliary means in a nonfloating position in the lower portion of the tank and to vent such vapor away from the space immediately above the floating roof. There is also a need to allow air from the atmosphere, or some other gas, to enter the space below the internal floating roof and above the level of product when the floating roof is supported in a nonfloating position. If air or some other gas were not allowed to enter this space a vacuum would develop, causing the floating roof to collapse into the space.

According to the invention, there is provided an improvement in a storage tank for a volatile liquid having an outer roof and an internal floating roof. The improvement comprises a vent pipe having one end in liquid communication with the bottom space of the tank and the other end extending to outside of the tank for venting vapor and air, or some other gas. Advisably the vapor and air or other gas beneath the floating roof is vented to outside of the tank, such as to the atmosphere or a vapor storage means. The tank has means to support the floating roof above the tank bottom and above the liquid when the liquid contents drops below a predetermined level. The vent pipe has one or more openings or port means conducting vapor or air between the space below the floating roof, when the floating roof is supported in nonfloating position above the liquid level, and the atmosphere outside of the tank or a vapor storage means. Also, advisably included is a float in the vent pipe which floats on liquid therein from the tank. The float retards evaporation of liquid in the vent pipe when the liquid level floats the floating roof. When the liquid level is too low to float the floating roof, the float is positioned below the openings or port means in the vent pipe to permit flow of vapor or air between the space below the floating roof and the atmosphere outside of the tank. When the tank is being filled and the floating roof passes from the nonfloating stage to the floating stage, the vent pipe will be filled with vapor. At some time during the venting of this vapor to the atmosphere it will pass through the explosive range. The float insulates the surface of the liquid in the vent pipe from thi explosive mixture. In the event that a source of ignition is introduced into this vapor, the resulting explosion should not ignite the liquid in the vent pipe, and should cause little or no damage to the pipe, the tank, or the contents.

The invention will be described further in conjunction with the attached drawings in which:

Flg. 1 is a vertical sectional view through a storage tank having a vent pipe according to the invention with the floating roof supported in its lowermost position and with the liquid level lower than the floating roof;

FIG. 2 is similar to FIG. 1 but shows the floating roof floating on liquid in the tank;

FIG. 3 is an enlarged elevational view, partially in section, of the lower part of the vent pipe and a portion of the floating roof as positioned in FIG. 1; and

FIG. 4 is an enlarged elevational view, partially in section, showing the lower portion of the vent pipe with the floating roof floating on liquid in the tank.

So far as is practical, the same parts or elements which appear in the different views of the drawings will be identified by the same numbers.

With reference to FIG. 1, storage tank 10 has a metal bottom 11, a vertical circular metal wall 12 and a domed outer roof l3. Conduit 14 is for supplying liquid to the tank and for removing it therefrom. Floating roof 15 is a metal pan with a vertical peripheral edge and is designed to float on liquid in the tank when the liquid level rises above supports 16. Supports 16 are connected to the inner surface of tank waLl 12. As shown in FIG. 1, floating roof l5 rests on supports I6 above the tank bottom since the liquid level 17 is below such supports.

Vent pipe 18 is conveniently positioned in tank 10 and extends vertically from the inner surface of tank bottom 11, to which it is connected, through floating roof l5 and through outer roof 16 where it communicates with the atmosphere. Cap 19 extends over the upper or outer end of vent pipe 18 to prevent rain and snow from entering the vent pipe. However, cap 19 is raised above the upper end of vent pipe 18 to provide space for vapor to flow out, or air to flow in.

The bottom end of vent pipe 18 has holes 20 therein adjacent to the tank bottom so that liquid in the tank can flow into or out of the vent pipe. The liquid level in vent pipe 18 therefore will always be the same as the liquid level elsewhere inside the tank. One or more port means or holes 21 are located in vent pipe 18 at a location below floating roof when it rests on supports 16. The holes are located in close proximity to the bottom of the floating roof. Vapor which accumulates beneath floating roof 15 when the liquid level in the tank drops to a level which results in the floating roof resting on supports 16 in a nonfloating position, flows through holes 21 into vent pipe 18 through which it flows upwardly and is vented to the atmosphere. Similarly, air will be drawn into the space below the floating roof as the liquid level drops below that at which the roof 15 lands on its supports 16 when liquid is withdrawn from the tank.

The portion of floating roof 15 through which the vent pipe 18 passes has a vertical circular wall 22 from which sealing member 23 extends inwardly into contact with vent pipe 18 to provide a slidable seal therebetween so that vapor cannot readily flow from beneath the floating roof into the space thereabove. Vertical wall 22 and seal means 23 also serve to define a stilling chamber 24 therebeneath for accumulating vapor before it flows through openings 21. This serves to minimize liquid entrainment during the last stages of the escape of vapor from beneath the floating roof whn the tank is being filled with liquid.

FIG. 2 illustrates the floating roof as it floats on liquid in tank 10. There is no need to provide for escape of vapor through holes 21 when the liquid level in the tank is above such holes since there is no vapor beneath the floating roof when it is floating. Furthermore, vapor can be prevented from escaping through vent pipe 18 even though the liquid level in vent pipe 18 is essentially the same as the bottom of floating roof 15 by placing float 25 inside of the vent pipe. Float 25 floats on the liquid in the vent pipe and is sufficiently wide to essentially contact the inner surface of the vent pipe to thereby substantially retard vaporization of liquid beneath it. It should be understood that the vent pipe can be suitably employed in some tanks for the storage of some liquid without employing a float 25 in the vent me. p FIG. 3 illustrates in greater detail the lower portion of vent pipe 18 when the floating roof rests on supports 16 and the liquid level 17 is below such supports and does not float the floating roof. The floating roof 15, as shown in FIG. 3, is positioned as it would be when it rests on supports 16. At such position, holes 21 are located beneath the floating roof and, in this instance, that means below seal means 23. Holes 21 provide means for vapor to flow from the space beneath the floating roof into vent pipe 18 and through it to the atmosphere, and for air to flow from the atmosphere through pipe 18 into the space below the roof. Furthermore, by having holes 21 beneath the floating roof, the vapor is prevented from flowing through such holes and out others into the space above the floating roof but inside of the tank. Float 25 drops with the liquid level in the tank until it becomes supported by rods 26 which project inwardly from the inside of vent pipe 18. When so supported, float 25 is located below ports or holes 21 so that it does not interfere with vapor flow through such holes. Although float 25 could be permitted to drop further with further lowering of the liquid level in the tank, it is advisable to prevent the float from dropping completely to the bottom of the tank so as to keep it always above whatever sludge or impurities might deposit on the tank bottom.

When liquid is fed into tank 10 by conduit 14, float 25 is raised from rods 26 and begins to move by floatation upwardly inside of vent pipe 18. Float 25 continues such upward ascent floating on the liquid inside of the vent pipe as liquid is fed to the tank. The float however does not interfere with vapor passage from beneath the floating roof into the vent pipe as it rises. When the liquid level in the tank finally floats the floating roof, float 25 will be positioned approximately at the top of the uppermost holes 21. As the floating roof rises with the liquid level in the tank, it moves above holes 21, as does the liquid level, so that there is no longer vapor which must be vented from beneath the floating roof. FIG. 4 illustrates the position of the floating roof and of the float when they are each floating on liquid in the tank at a level above holes 21.

When liquid is withdrawn from the tank 10 through conduit 14 the floating roof 15 and the float 25 descend until the roof 25 comes to rest on supports 16. At this point the liquid level 17 drops below the bottom of the roof 15, and the float 25 drops below the uppermost holes or port means 21. Air then enters the space between the roof l5 and the surface of the liquid 17 through holes 21, thus preventing development of a vacuum in the space.

Although the invention has been described with reference to a noninsulated storage tank and a noninsulated floating roof, it is obvious that the vent pipe system as described herein can be employed in an insulated storage tank, with an insulated or noninsulated floating roof. in addition, float 25 can be insulated or noninsulated depending upon the liquid and conditions of storage employed.

The foregoing detailed description has been given for clearness of understanding only, and no unnecessary limitations should be understood therefrom, as modifications will be obvious to those skilled in the art.

What is claimed is:

1. in a storage tank for a volatile liquid having an outer roof and an internal floating roof, the improvement comprising:

a vent pipe having one end in liquid communication with the bottom space of the tank and the other end extending in vapor communication to outside the tank;

means to support the floating roof above the tank bottom and above the liquid when the liquid contents drops below a predetermined level;

a vapor vent opening in the vent pipe beneath and in close proximity to the bottom of the floating roof when it is in nonfloating position; and

a float in the vent pipe, said float having its peripheral edge in close proximity to the inner surface of the vent pipe.

2.'The improvement of claim 1 in which the vent pipe is vertically supported on the tank bottom and extends through the tank to the outside thereof.

3. The improvement of claim 2 in which the vent pipe has an opening near the bottom of the tank for liquid flow thereto.

4. The improvement of claim 1 in which the floating roof and float are insulated.

5. The improvement of claim 1 in which the vent pipe has means to support the float in non-floating position lower than the vapor vent opening but above the tank bottom. 

1. In a storage tank for a volatile liquid having an outer roof and an internal floating roof, the improvement comprising: a vent pipe having one end in liquid communication with the bottom space of the tank and the other end extending in vapor communication to outside the tank; means to support the floating roof above the tank bottom and above the liquid when the liquid contents drops below a predetermined level; a vapor vent opening in the vent pipe beneath and in close proximity to the bottom of the floating roof when it is in nonfloating position; and a float in the vent pipe, said float having its peripheral edge in close proximity to the inner surface of the vent pipe.
 2. The improvement of claim 1 in which the vent pipe is vertically supported on the tank bottom and extends through the tank to the outside thereof.
 3. The improvement of claim 2 in which the vent pipe has an opening near the bottom of the tank for liquid flow thereto.
 4. The improvement of claim 1 in which the floating roof and float are insulated.
 5. The improvement of claim 1 in which the vent pipe has means to support the float in non-floating position lower than the vapor vent opening but above the tank bottom. 